1. Field of the Invention
The present invention relates to a technology for correcting a pixel value of a target pixel in image data.
2. Description of the Related Art
In image display apparatuses having brown tube display or liquid crystal display and in which an image is displayed in pixels, undershoot or overshoot occurs when an edge enhancement is performed to sharpen the edges of the displayed image. In overshoot the image has a white border upon edge enhancement and in undershoot the image has a black border upon edge enhancement. There have been various attempts to reduce the overshoot and undershoot caused by edge enhancement.
For example, in an image correcting circuit disclosed in Japanese Patent No. 3326377, pass filtering is applied to an input image according to a quantity of high-frequency components, making the degree of edge enhancement variable according to the quantity of high-frequency components, thus suppressing overshoot or undershoot and achieving edge enhancement according to the sharpness of the edges of the input image.
In Japanese Patent Application Laid-open No. 2003-204451, a device and method are disclosed in which Haar wavelet transform method is used for reducing overshoot and undershoot at the edges and to enhance the sharpness of the edges of the display image. Haar wavelet transform a type of wavelet transform and is the simplest of the wavelet transform in which wavelets that are enlarged, compressed and parallel-shifted are superimposed to express all possible waveforms.
In Japanese Patent Application Laid-open No. 2005-333530, an image signal processing apparatus is disclosed which receives a plurality of image signals near a target pixel of an input image signal as an input and extracts the signal of the target pixel, at least two pixels before and after the target pixel, and pixels for secondary differentiation further before and after the previous set of pixels. The image signal processing apparatus outputs a secondary differential signal by subjecting the plurality of extracted pixels to a secondary differentiation, and adds the secondary differential signal to the image signal of the target pixel to form a sum signal. The image signal processing apparatus then detects a maximum pixel value and a minimum pixel value of at least three pixels that include the target pixel and the signals before and after the target pixel and selects a median from among at least three signals that include the output signal, the signal with the maximum pixel value and the signal with the minimum pixel value. Consequently, the image signal processing apparatus suppresses overshoot and undershoot, and performs outline correction to satisfaction even if the input signal has an undulating outline and generates good image signals.
However, in the conventional technologies described above, it is difficult to maintain the sharpness of the edges while suppressing overshoot and undershoot.
Specifically, in the technologies disclosed in Japanese Patent No. 3326377 and Japanese Patent Application Laid-open No. 2003-204451, the sharpness of the edges varies according to the degree of suppression of overshoot-undershoot. Thus, if the degree of suppression of overshoot-undershoot is made mild, the sharpness of the edges is reduced, and if the sharpness of the edges is increased, the degree of overshoot-undershoot suppression is emphasized. Therefore, it is difficult to achieve both sharpness of the edge and overshoot-undershoot suppression at the same time.
In the technology disclosed in Japanese Patent Application Laid-open No. 2005-333530, a little overshoot-undershoot is necessary to achieve edge enhancement. Consequently, contour processing needs to be done twice, the first time followed by overshoot-undershoot removal and the second time followed by overshoot-undershoot generation. Thus, if the input image has a smooth contrast variation, the resulting image after two rounds of edge enhancement loses contrast and has a jaggy (aliasing) appearance with a sharp contour.